Process for producing DHA-containing tofu or soybean milk drink, or dry powder thereof

ABSTRACT

The present invention provides a process for producing DHA (docosahexaenoic acid)-containing tofu or soybean milk drink, or a dry powder thereof which does not influence taste even if a DHA-containing fish oil is incorporated at a high concentration, which is stable and of which starting materials are inexpensive. A DHA-containing fish oil emulsion is prepared by mixing soybean milk with a DHA-containing fish oil at a weight ratio of 1:0.2 to 1, and stirring the mixture. Then, soybean milk drink can be produced by further diluting this emulsion with soybean milk to give a predetermined DHA concentration. Still further, DHA-containing tofu can be produced by coagulating the soybean milk containing this emulsion with the addition of a coagulant. Since the same soybean milk as the starting material of tofu or soybean milk drink is used as an emulsifying agent, such a product is obtained that even if DHA is contained at a high concentration, this does not influence a taste or properties of the product. A dry powder can be obtained by freeze-drying, vacuum-drying or spray-drying this tofu or soybean milk drink as required.

This application is a 371 application of International Application No.PCT/JP97/00925 filed Mar. 21, 1997.

FIELD OF THE INVENTION

The present invention relates to a process for producing tofu (beancurd), soybean milk drink or a dry powder thereof which contains DHA(docosahexaenoic acid) at a high concentration and which does not havean offensive smell of a fish oil.

BACKGROUND ART

DHA is one of n-3 highly unsaturated fatty acids that in recent years,have attracted attention as essential fatty acids having a wide varietyof physiological activities. DHA has been known to have anantithrombogenic activity, an antiinflammatory activity, an antiallergicactivity and an antitumor activity and to be important in developing andmaintaining a nerve tissue of brain, retina or the like; it hastherefore aroused interest. DHA is contained in a fish oil extractedfrom fish and shellfish in a relatively large amount. Such aDHA-containing fish oil has been marketed as a capsule-type healthy foodformed by coating the same with gelatin or the like. Meanwhile, attemptshave been made to add the same to general foods such as a beverage, aconfectionery, a dairy dish, a seasoning and the like, and there are alarge number of commercial products.

However, since DHA has a long chain and many double bonds and gives apeculiar smell derived from a fish oil, the purification, theconcentration and the anti-oxidation thereof are technically difficultin many cases. As a result, a DHA-containing fish oil which is subjectedto purification, emulsification, milling and anti-oxidation such asaddition of an antioxidant is costly. For this reason, DHA is currentlyadded to food only in an amount which is far from an average amount ofDHA (from 0.5 g to 1.0 g per day) ingested by a Japanese from fish andshellfish. Further, in the conventional technique, after theDHA-containing fish oil emulsified or milled is added to a food, thestability of DHA is not maintained satisfactorily, so that such a foodcannot be stored. Thus, the addition of DHA at a high concentration isprevented.

On the other hand, soybeans abundantly contain proteins having a highessential amino acid content and excellent in vivo absorbability,linoleic acid which is an n-6 highly unsaturated fatty acid, andfunctional substances such as lecithin and saponin having acholesterol-decreasing activity owing to their emulsifiability. Further,soybeans, as well as garlic, onion and the like, have drawn attention asa food having an activity to prevent tumor. Tofu or soybean milk drinkwhich is produced using such soybeans as a starting material naturallycontains these useful substances abundantly, and is eaten and drunkevery day as a traditional healthy food in Japan.

If DHA can be added to the above-mentioned tofu or soybean milk drinkwhich is the typical healthy food in Japan, a person who does not eatfish frequently can naturally ingest DHA from a dairy meal or drink, andhis health can be expected to thereby be maintained or promoted. It isconsidered that DHA is added to tofu or soybean milk drink by a methodin which an emulsion having a high storage stability, which is obtainedby emulsifying a DHA-containing fish oil using an appropriateemulsifying agent and adding a preservative such as sugar or salt at ahigh concentration to increase a storage stability, is added; a methodin which the above-described emulsion is added as a dry powder; or amethod in which a powder obtained by adsorbing a DHA-containing fish oilonto a porous carrier is added.

However, there has been no report indicating that DHA is added to tofuor soybean milk drink in such an amount that a Japanese can ingest DHAin a day from fish or shellfish, nor has the commercial product beenmarketed. The reason is that not only the above-described emulsion, itspowder or porous support-adsorbed powder of the DHA-containing fish oilis expensive relative to the DHA content, thereby increasing the costsof the starting materials for tofu or soybean milk drink, but also theemulsifying agent, the preservative, the porous carrier and the likewhich are used influence the taste and the properties of tofu or soybeanmilk drink, thereby preventing the addition of DHA at a highconcentration.

DISCLOSURE OF THE INVENTION

Accordingly, an object of the present invention is to provide a processfor producing DHA-containing tofu or soybean milk drink, or a dry powderthereof which does not influence taste even if a DHA-containing fish oilis incorporated at a high concentration, which is stable and of whichstarting materials are inexpensive.

A process for producing DHA-containing tofu in accordance with thepresent invention comprises mixing soybean milk with a DHA-containingfish oil at a weight ratio of 1:0.2 to 1, stirring the resulting mixtureto form a DHA-containing fish oil emulsion, diluting this emulsion withsoybean milk, and then coagulating the resulting dilute with theaddition of a coagulant.

A dry powder of DHA-containing tofu can be produced by furtherdry-milling the above-obtained tofu.

Still further, a process for producing DHA-containing soybean milk drinkin accordance with the present invention comprises mixing soybean milkwith a DHA-containing fish oil at a weight ratio of 1:0.2 to 1, stirringthe mixture to form a DHA-containing fish oil emulsion, and dilutingthis emulsion with soybean milk.

A dry powder of DHA-containing soybean milk can be produced by furtherdry-milling the above-obtained soybean milk drink.

The term "a DHA-containing fish oil" in the present specification isused as a general term of an oil which is extracted from fish andshellfish in a usual manner, and partially purified through purificationsteps such as deoxidation, decoloration, deodorization and the like andwhich is composed mainly of triglycerides including highly unsaturatedfatty acids such as DHA, EPA (eicosapentaenoic acid) and the like.

An emulsifying agent and a preservative which are ordinarily used inproducing a DHA-containing fish oil emulsion have an adverse effect on aflavor of tofu in many cases. However, in the present invention, sincethe same soybean milk as the starting material of tofu or soybean milkdrink is used as an emulsifying agent to emulsify a DHA-containing fishoil. Therefore, even when tofu or soybean milk drink is produced byadding the resulting emulsion in a large amount to soybean milk, aproduct containing DHA at a high concentration can be produced withoutimpairing the flavor and the properties of the tofu or the soybean milkdrink. Furthermore, in accordance with the present invention, eventhough a slight fish smell is given out in a DHA-containing fish oil, itis masked by emulsifying this oil with soybean milk, with the resultthat tofu or soybean milk drink obtained by adding this emulsion doesnot have a fish smell, and it is almost the same as that obtainedwithout the addition of the emulsion.

BEST MODE FOR CARRYING OUT THE INVENTION

In producing DHA-containing tofu according to the present invention,soybean milk used as a starting material of tofu and also as anemulsifying agent is first prepared. The preparation of the soybean milkcan be carried out by employing a method which has been so far usedconventionally. That is, soybeans are dipped in water overnight, andwater is further added thereto. The soybean/water mixture is ground toobtain a slurry (gojiru). This slurry is heated, and then filtered toseparate a refuse (okara) as a solid content to obtain soybean milk as aliquid.

A part of this soybean milk is used as an emulsifying agent. Thissoybean milk and a DHA-containing fish oil are mixed at such a ratio(weight ratio) that the soybean milk is 1 while the fish oil is between0.2 and 1, and the mixture is stirred to prepare an emulsion of theDHA-containing fish oil. This emulsification procedure can be conductedusing a relatively small-sized stirrer, and besides the oil content ishigh, with the result that bubbling little occurs in the stirring andeffective emulsification can be conducted without the use of anexpensive stirrer fitted with a deaerator.

The thus-obtained emulsion containing DHA at a high concentration isdiluted with the addition of soybean milk to adjust DHA to apredetermined concentration, and the resulting dilute is gently stirredand dispersed. Then, the resulting dispersion is coagulated with theaddition of a coagulant such as magnesium chloride in a usual manner toproduce tofu. Silken tofu, pressed silken tofu or packed silken tofu canbe produced by slightly changing the coagulation procedure in a usualmanner.

Also when DHA-containing soybean milk drink is produced according to thepresent invention, a soybean milk which is used as a starting materialof soybean milk drink and also as an emulsifying agent is firstproduced. The soybean milk which is a starting material of soybean milkdrink can also be produced in the same manner as the above-descrobedsoybean milk for tofu. However, a process for producing soybean milkdrink which is currently mass-produced mostly includes a step ofdeactivating lipoxygenase that gives a soybean milk smell and a vacuumtreatment step for deodorization. That is, the enzyme is deactivated byjetting high-pressure water vapor to peeled soybeans. Then, theresulting soybeans are mixed with hot water and ground using grinder toform a slurry. Thereafter, a refuse as a solid content is separatedusing a centrifugal separator or a filter to obtain soybean milk.

A part of this soybean milk drink is used as an emulsifying agent. Thissoybean milk and a DHA-containing fish oil are mixed at such a ratio(weight ratio) that the soybean milk is 1 while the fish oil is between0.2 and 1, and the mixture is stirred to prepare an emulsion of theDHA-containing fish oil. This is the same as the above-mentioned processfor producing tofu. In this case as well, the emulsification procedurecan be conducted using a relatively small-sized stirrer, and besides theoil content is high, with the result that bubbling little occurs in thestirring and effective emulsification can be conducted without the useof an expensive stirrer fitted with a deaerator. The thus-obtainedemulsion containing DHA at a high concentration is diluted with theaddition of soybean milk to adjust DHA to a predetermined concentration,making it possible to produce a soybean milk drink product. A usualcommercial soybean milk drink product is produced by further sterilizingthe product for several tens of seconds, then deaerating the sterilizedproduct under reduced pressure, homogenizing the resulting product usinga high-pressure emulsifier, and packing it into a container.

The DHA-containing tofu obtained in accordance with the presentinvention can be eaten, like the conventional tofu, as chilled tofu,simmering tofu, miso soup or the like. The DHA-containing soybean milkdrink obtained in accordance with the present invention can also bedrunk like the conventional drinking soybean milk. Further, theDHA-containing tofu or soybean milk drink can be dried and milled byusing, for example, freeze-drying, vacuum drying, spray drying or thelike. The dry powder obtained is a functional food material containingDHA, EPA, a soybean protein and a soybean oil, and it can be used as aprocessed food material or a healthy food through tableting or the like.

The present invention is illustrated more specifically by referring tothe following Examples and Comparative Example.

EXAMPLE 1

Ten kilograms of soybeans were dipped in water for 12 hours, and groundwith the addition of water to obtain 55 liters of a slurry. This slurrywas heated at 100° C. for 5 minutes, and then filtered to obtain 45liters of soybean milk as a liquid.

Two kilograms of the resulting soybean milk and 1 kg of a DHA-containingfish oil (DHA content 23 wt. %) having a slight fish smell were mixed,and stirred at 7,500 rpm for 10 minutes using a homomixer (T. K.Homomixer MARK II, supplied by Tokushu Kika Kogyo K. K.) to give aDHA-containing fish oil emulsion (DHA content approximately 7.7 wt. %).

The above-mentioned soybean milk cooled to 10° C. was divided intoportions each having a weight of 10 kg. The above-mentioned emulsion wasadded thereto in an amount of 0 g, 400 g, 800 g or 1,600 g, and theywere stirred and mixed using a spatula. Subsequently, 0.25 wt. % ofmagnesium chloride was added thereto as a coagulant. A predeterminedamount of the mixture was charged into a container for packed silkentofu, and the container was sealed. This container was heated at 90° C.for 40 minutes, and then cooled to obtain DHA-containing tofu.

On the production day, a panel test or sensory test (panelist: 3 men and2 women) was conducted. Consequently, no fish small was given even inthe maximum concentration-added sample (sample with the addition of1,600 g of the emulsion), and the evaluation was that none of the addedsamples was different from the non-added sample.

After the resulting tofu was stored at 10° C. for 7 days, the sensoryevaluation was conducted. The evaluation was that there was nodifference between the added samples and the non-added sample. Thus, agood stability was shown.

On the production day and after 7 days of the storage at 10° C., a misosoup and simmering tofu were made by using the resulting DHA-containingtofu, and compared with those made by using DHA-free tofu. As a result,the DHA-containing tofu was not different at all from the DHA-free tofuin the color, the smell, the taste and the feeling upon eating.

The tofu obtained by mixing 10 kg of the soybean milk with 1,600 g ofthe emulsion contained approximately 1.1 g of DHA per 100 g of the tofu.Since a standard weight of a commercially available piece of tofu isapproximately 300 g, an average amount of DHA ingested by a Japanese ina day from fish and shellfish can be satisfied well by eating 1/3ofapiece of tofu.

COMPARATIVE EXAMPLE 1

A DHA-containing fish oil emulsion (DHA content approximately 7.5 wt. %)was prepared using the same DHA-containing fish oil as that used inExample 1, water, sodium caseinate as a commercial emulsifying agent andlecithin.

The soybean milk prepared in the same manner as in Example 1 was dividedinto portions each having a weight of 10 kg. The above-mentionedemulsion was added thereto in an amount of 0 g, 100 g, 200 g or 500 g,and they were stirred and mixed using a spatula. Subsequently, 0.25 wt.% of magnesium chloride was added thereto as a coagulant. Apredetermined amount of the mixture was charged into a container forpacked silken tofu, and the container was sealed. This container washeated at 90° C. for 40 minutes, and then cooled to obtainDHA-containing tofu.

As a result of a panel test, a test sample in which 100 g of theemulsion were added to 10 kg of the soybean milk was scarcely differentfrom the non-added sample. However, in the test sample in which 200 g ofthe emulsion were added thereto gave a slight fish smell. In the testsample in which 500 g of the emulsion were added thereto gave a clearfish smell, and an unpleasant taste was provided. Further, the higherthe concentration of the emulsion, the softer the tofu became.

The tofu obtained by adding 200 g of the emulsion to 10 kg of the tofucontained approximately 0.15 g of DHA per 100 g of the tofu. However,the amount of DHA which could be added was small as compared with theresults in Example 1.

EXAMPLE 2

Three kilograms of the tofu obtained in Example 1 by mixing 10 kg ofsoybean milk with 1,600 g of the emulsion were mashed on a stainlesssteel plate using a spatula, and then subjected to freeze-drying. Theresulting dry product was milled using a food processor to obtain 488 gof a DHA-containing tofu dry powder. One hundred grams of this powderhad an oil content of 48 g, and 6.5 g of 48 g of the oil content wasDHA. The thus-obtained DHA-containing tofu dry powder was completelyfree from a fish smell, and the taste thereof was found to be almost thesame as that of a powder obtained by freeze-drying DHA-free tofu.

EXAMPLE 3

High-pressure water vapor was jetted to 10 kg of peeled soybeans for 2minutes to deactivate an enzyme, and then ground with 50 kg of hot waterof 90° C. using a grinder. The resulting slurry was filtered to obtain30 kg of soybean milk. Two kilograms of this soybean milk were mixedwith 1 kg of a DHA-containing fish oil in the same manner as in Example1, and the mixture was stirred at 7,500 rpm for 10 minutes using ahomomixer to obtain a DHA-containing fish oil emulsion.

Two kilograms of this emulsion were added to 20 kg of the above-obtainedsoybean milk, and they were gently stirred and mixed to obtain soybeanmilk containing approximately 0.7 wt. % of DHA. Further, thethus-obtained soybean milk was indirectly sterilized for several tens ofseconds using a plate heater, deaerated using a reduced pressure tankand homogenized (treatment pressure: 200 kg/cm²) using a high-pressureemulsifier (Laboratory Homogenizer, supplied by APV-Gaulin, Inc. inU.S.A.) to obtain 20 kg of DHA-containing soybean milk drink having asolid content of 9 wt. %. The resulting soybean milk drink was subjectedto a panel test along with commercial soybean milk drink. As a result,it was evaluated that there was no fish smell and the flavor of theabove-obtained soybean milk was equal to that of the commercial soybeanmilk.

EXAMPLE 4

Ten kilograms of the soybean milk drink obtained in Example 3 werespray-dried to obtain 750 g of a DHA-containing soybean milk dry powder.This dry powder was a yellowish white powder having a flavor of soybeanmilk. Even after 1 month of storage in a sealed container at 27° C., nofish smell was given out, and a good flavor was maintained.

INDUSTRIAL APPLICABILITY OF THE INVENTION

As is understood from the foregoing, in accordance with the presentinvention, tofu or soybean milk drink is produced such that aDHA-containing fish oil emulsion in which the same soybean milk as astarting material of tofu or soybean milk drink is used as anemulsifying agent is diluted with soybean milk to a predetermined DHAconcentration. Consequently, even if DHA is contained at a highconcentration, tofu or soybean milk drink can be obtained withoutexerting any influence on a taste or properties.

Especially, soybean milk is used as an emulsifying agent, whereby a fishsmell peculiar to a DHA-containing fish oil is masked. As a result, notonly tofu or soybean milk drink free from a fish smell can be produced,but also is a cost for an emulsifying agent dispensed with because thereis no need to use an ordinary commercial emulsifying agent.

In addition, in view of a production apparatus or a production step, a2-step process comprising an emulsification step (first step) ofobtaining a DHA emulsion having a high DHA concentration by usingsoybean milk as an emulsifying agent and a dilution step (second step)of diluting the resulting emulsion with the same soybean milk to give apredetermined DHA concentration is employed, so that a relativelysmall-sized stirrer can be used in the first step of the emulsification.Besides, since the oil content can be increased by adding theDHA-containing fish oil at a high concentration, bubbling little occursduring the stirring. The resulting emulsion is further diluted with alarge amount of soybean milk in the second step, so that bubbles almostdisappeared, and there is no fear of incorporating bubbles in producingtofu.

On the contrary, when a step of diluting a DHA-containing fish oil withsoybean milk to give a predetermined DHA concentration and anemulsification step are conducted simultaneously in one step, alarge-sized emulsifier is required.

In practizing the above-described process of the present invention, itis sufficient that an existing tofu production apparatus or soybean milkdrink production apparatus is fitted with a small-sized emulsifier, andthat a DHA-containing fish oil emulsion prepared with the emulsifier isdirectly introduced into the existing tofu production apparatus orsoybean milk drink production apparatus. Consequently, equipmentinvestment or a cost required for change of steps can be minimized.

The dry powder of the DHA-containing tofu or the soybean milk drink canwidely be used as a functional food material containing DHA, EPA, asoybean protein and a soybean oil.

We claim:
 1. A process for producing DHA (docosahexaenoic acid)-containing tofu, which comprises mixing soybean milk with a DHA-containing fish oil at a weight ratio of 1:0.2 to 1 to produce a mixture, emulsifying the mixture to form a DHA-containing fish oil emulsion, diluting this emulsion with soybean milk to produce a diluted emulsion, and then coagulating the diluted emulsion with the addition of a coagulant.
 2. A process for producing a dry powder of DHA-containing tofu, which comprises further dry-milling the tofu obtained in claim
 1. 3. A process for producing DHA (docosahexaenoic acid)-containing soybean milk drink, which comprises mixing soybean milk with a DHA-containing fish oil at a weight ratio of 1:0.2 to 1 to produce a mixture, emulsifying the mixture to form a DHA-containing fish oil emulsion, and diluting this emulsion with soybean milk.
 4. A process for producing a dry powder of DHA-containing soybean milk, which comprises further dry-milling the drinking soybean milk obtained in claim
 3. 